20~30 souls total for a raid event...? That's definitely not right, unless you're putting no time into the game. That's like saying you only run like ~30 raids total the entire 7~8 day event. Many players have to run over a hundred or two just for soul rewards. Raid events are excruciatingly time consuming, save all your BP recovery seeds to run them. If you're weak, that's fine, you get souls for participating and raid events don't need to be done solo. Advent events are very unfriendly towards new players, don't expect to complete the event eidolon until you can farm Ultimate or willing to put a lot of time and maybe even money into farming Expert (if you don't have the elixirs to do it)

Grimoires are gotten from events. So yes, the more you play the more you'll get. You'll eventually get enough that they just start stock piling. You can even get them in the raid event gachapon but when you're strong enough to handle Ultimate for Advent events, you can generally grab several SR and SSR grimoires without much issue.

For leveling, high rank players can just run the hardest currently available content like Ultimate Advent quest with their hime they need to level on the party sub slots without ever needing to use them. Either that or farm Sunday SP quest for lot of EXP. Since you're low level, you don't have much choice but to just keep battling with what you got and slowly progress til you have a decent level party to power level your other units.